I think this has been shuffled around here before, but I don't recall any very definitive answers. As a learning tool, I would love to hear from the working electricians as to how they would proceed.

Here is the situation-

You have been dispatched to newly built home. The complaint is that the circuit breaker for the bathroom trips. In the home you find that the bathroom receptacles and lighting are powered by a dedicated 20A circuit which is being fed by a GFCI breaker. The receptacles and lights are working. Any motor load such as a hair drier or vaccuum cleaner will trip the breaker. About 20% of the time, flipping on the lights (4 60w cans) alone on will trip the breaker. The homeowner states that it has been doing this since they moved in.

As a test, a standard non-GFCI breaker was temporarily installed and found to operate properly without tripping. You do not have another GFCI breaker on hand.

As a professional technician, what steps would you begin with in diagnosing this problem?

Are you sure it is a GFCI breaker feeding the bath or an AFCI breaker? Just checked NEC and AFCI not required for bathroom.
Without being there it be hard for me to explain.But put an amp meter on the curcit with the non GFCI breaker and put the full load on it.

Probably a bad breaker. Like suggested, check with a ammeter to be sure...

Assuming the house has more than one bathroom, there should be another 20A GFCI breaker in the panel that you could borrow temporarily to see if it also trips.

If properly wired, either the bathroom receptacles are fed by a dedicated circuit and the lights are on some other circuit, or each bathroom has it's own dedicated circuit.

edit: Since this seems to be a test more than a question, you could connect another circuit to the breaker in question and see if it still trips (with a known good resistive load like a work light or something...). Also, completely disconnect the bathroom circuit and check for resistance to ground (should be infinite), especially on the neutral side -- after checking for voltage first, of course.

For years, I had an intermittent GFCI fault in my kitchen. IT might go months, or eventually, it tripped if you looked at it funny. I thought it was the GFCI, but it persisted after changing it. I then I started disconnecting things. I isolated it to a wire between two receptacles. With that wire out of the circuit, everything worked (well, except for that last receptacle!). Since it was going to be a pain rerunning that wire, I ended up feeding that last receptacle with a home run. I had planned to put a wine cooler on that one anyway, so a dedicated circuit was not a bad idea. What I'm saying, is that while it could be the GFCI, it could just as easily be a wire that is marginal somewhere, either inside of a device, or the wire itself. A nail, staple, etc. or a nick in the insulation somewhere is all it might take.

On the neutral side, a high resistance could cause problems only under load -- at least in theory.

Without a lot of digging into the details myself first, I can't really say much more without risking saying something foolish/wrong. I have seen Square-D (homeline) GFCI breakers trip from someone touching the neutral on a light fixture with the switch off -- very strange.

One other idea I just had is that some electronic controls (timer/dimmer) are (according the the mfr directions) supposed to be connected to the ground instead of the neutral. I guess a few milliamps won't hurt anything in a non-GFCI circuit when the neutral isn't available, but it seems like it could be problematic on a GFCI-protected circuit...

Dave, breaking the circuit into sections is good troubleshooting. Thanks for that thought.
JW, I understand when a GFCI is supposed to trip. The topic is how to troubleshoot the fault. When the symptom of the problem is easily reproduced, how can it be a "sporadic" neutral to ground short?
HJ, your statement is acknowledged. The neutral is not shared.

JW, I understand when a GFCI is supposed to trip. The topic is how to troubleshoot the fault. When the symptom of the problem is easily reproduced, how can it be a "sporadic" neutral to ground short?

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When putting everything into a box such as a receptacle or switch (not so much with a switch) a contact with the bare EGC will cause a GFCI device to open. If the contact is just barley touching it is very possible that the contact only makes when the device has pressure applied such as with the flipping of a switch or plugging or unplugging of a cord.

It would be my guess without being there that there is a staple driven too tight that makes the contact only under a heavy load when there is expansion of everything due to the heat applied to the conductors. Trouble shooting anything such as this can be a brain teaser and very hard to find even for the most experienced of electricians.

The bottom line is simple, the device is doing what it was designed to do and to replace it with a device that is not GFCI protected will result in some very bad things. It may take a while before these bad things starts happening but they will happen.

JW, I appreciate your input, but still, I am waiting to hear what methods you might use to track down the problem.

Don, the scope could certainly show some anomalies that a simple meter would not. As interesting as it might be, I try to stay away from the rocket science. I don't think many residential electricians are running around with a scope.

This was a problem that I resolved some time ago. I am still fishing for better ways to diagnose such faults. It's pretty easy to locate the source of this kind of problem when the circuits are exposed. It becomes much more challenging for me when I am looking at a finished room and don't know how the circuit is wired.

In retrospect, I am not so much looking for theory or possibilities. I am interested in hearing what regular electricians do when they walk into a job like this. What testing tools do you grab and where do you start?

I am surprised that no one suggested using a megger to test the wiring. In outdoor equipment, ground faults are very often caused by moisture and poor or damaged insulation. Maybe this is not a common tool for most electricians?

Sometimes, the only rational way to isolate the problem is to start disconnecting things until the remaining circuit works properly. Once you've isolate a branch, continue with disconnecting things or bypassing individual items until you're only left with one. While a GFCI device can fail, if it turns on and trips when you hit the test button, while not an absolutely 100% check, it's close...swapping it with a new one where the same thing happens is pretty much 100%. Then, it's back to trying to isolate the problem. First thing is to carefully open and do a very thorough visual inspection of everything that's accessible. Understanding how the things works helps. You'd never want the neutral and ground tied together after the panel.

JW, I appreciate your input, but still, I am waiting to hear what methods you might use to track down the problem.

Don, the scope could certainly show some anomalies that a simple meter would not. As interesting as it might be, I try to stay away from the rocket science. I don't think many residential electricians are running around with a scope.

This was a problem that I resolved some time ago. I am still fishing for better ways to diagnose such faults. It's pretty easy to locate the source of this kind of problem when the circuits are exposed. It becomes much more challenging for me when I am looking at a finished room and don't know how the circuit is wired.

In retrospect, I am not so much looking for theory or possibilities. I am interested in hearing what regular electricians do when they walk into a job like this. What testing tools do you grab and where do you start?

I am surprised that no one suggested using a megger to test the wiring. In outdoor equipment, ground faults are very often caused by moisture and poor or damaged insulation. Maybe this is not a common tool for most electricians?

Click to expand...

Most real electricians should have a o-scope and a megometer available to them. A VOM does not tell all.

Handheld scopes are cheap and used for working on 3 phase systems and looking at line noise. Meggers are great for testing wire runs and true ground faults. You can also use a Ground Fault tester.

Many "residential electricians" are wire runners and do not have the proper tools for testing, but they can follow color codes and wiring drawings. No need to even read or speak English.